General and specific word knowledge as predictors of children's categorization

There is a debate in the literature as to whether young children’s categorization abilities are more closely related to their general productive vocabulary or their specific word knowledge. The present study examines this relationship in children 18 to 26 months of age. Specifically, it tests the hypothesis that children’s understanding of individual words is more closely related to their category performance than the overall size of their productive vocabulary. Twenty children at 19 months (range= 18-20 months) and 25 months of age (range= 24-26 months) participated in the study. Children’s category performance was assessed using an object manipulation task with novel objects. Half of the children received novel names for the novel objects, and half did not. Two measures were then assessed: children’s knowledge of the novel names in a comprehension test, and children’s productive vocabulary outside of the laboratory, as measured by a parental checklist. Consistent with previous findings, the results indicate that children with larger productive vocabularies show more advanced categorizing. More importantly, however, the data suggest that children who received names for the novel objects outperformed children who did not receive the names, regardless of the size of their productive vocabulary. These findings suggest that hearing object names help children categorize objects, and may be a better predictor of children’s category performance than their productive vocabulary. General and Specific 3 General and Specific Word Knowledge as Predictors of Children’s Categorization Children experience rapid advances in their cognitive and linguistic abilities toward the end of their second year. One important cognitive skill that appears during this time is their ability to categorize objects. Researchers have observed that when young children are presented with two distinct categories of objects in an object manipulation task, they may exhibit spontaneous sorting behavior in which they sequentially touch, and sometimes spatially group, first one and then the other set of objects (e.g. Riccuiti, 1965; Starkey, 1981; Sugarman, 1981). This dual sorting behavior, involving both classes of objects, indicates that children are able to recognize the similarities and differences between category members (Riccuiti, 1965; Sugarman, 1981). According to Sugarman (1981), this advanced sorting behavior indicates that children are able to “extend a class to a set of discrete elements.” Researchers suggest that this implies representational thought. That is, children are able to conceptualize that several things that exist are from the same category and are the same “kind” of thing (Riccuiti, 1965; Sugarman, 1981). Numerous researchers have used an object manipulation task to study age related changes in category development. Categorization performance is found to increase with age. For example, Starkey (1981) examined children’s categorization abilities at six, nine, and 12 months of age. He found that categorization behaviors were absent at six months of age, however by 12 months of age, children reliably sequentially touched three or four objects from one category. Also, he found that dual category behavior, in which children spatially group two categories of objects separately or sequentially touch both categories of objects, had not yet appeared at 12 months of age. Based on these results, Starkey suggested that selective manipulation of one class of objects emerged somewhere between General and Specific 4 nine and 12 months of age. Using the same task, Sugarman (1981) documented an increase in children’s abilities to dual categorize two classes of objects from 18 to 36 months. She found that by 36 months, children were reliably able to selectively touch or group both sets of objects. Additionally, Riccuiti (1965) found a sharp increase in advanced category behavior at 18 months of age, which he explained as possibly an effect of sampling variation or a methodological flaw. More recently, however, other researchers have documented a similar increase around this age (e.g. Gopnik & Meltzoff, 1987; Poulin-Dubois, Graham, & Sippola, 1995). Current research has focused on the relationship between cognitive development and concurrent increases in categorization abilities. One important milestone that usually occurs around 18 months of age is a period of accelerated vocabulary growth, known as the vocabulary spurt. During this time, children add new words to their productive vocabularies at an accelerated rate (Benedict, 1979). The vocabulary spurt is not an entirely universal phenomenon, but for most children it usually occurs between 18 to 20 months of age, or after they have over 50 words in their productive vocabularies (Gershkoff-Stowe & Smith, 1997; Goldfield & Reznick, 1990). Several researchers have investigated the developmental relationship between this linguistic advancement and concurrent developments in categorization. For example, Gopnik and Meltzoff (1987) reported a close temporal association between the development of advanced categorization skills and the onset of the vocabulary spurt. All of the children in their study demonstrated advanced category behavior before or at the same time as the onset of the vocabulary spurt, suggesting that children need to have achieved certain cognitive milestones in order to experience a vocabulary spurt. Further, General and Specific 5 they suggested that infants reflect the knowledge that objects belong in categories by naming and grouping objects. Poulin-Dubois, Graham, and Sippola (1995) also found that children experienced a significant improvement in categorization skills at the onset of the vocabulary spurt. Their findings additionally support Gopnik and Meltzoff’s (1987) claim that the advancement of these cognitive abilities is temporally related to the onset of the naming explosion. However, if these claims hold true, what are the implications for children who fail to demonstrate a rapid increase in their productive vocabulary? In Goldfield and Reznick’s (1990) longitudinal study, five out of 18 children did not demonstrate a vocabulary spurt. Were those five children unable to achieve advanced categorization skills? Research by Gershkoff-Stowe, Thal, Smith, and Namy (1997) suggests otherwise. They failed to replicate Gopnik and Meltzoff’s (1987) finding that the onset of the vocabulary spurt was strongly related to the emergence of advanced categorization skills. Instead, they suggested that increased categorization abilities should be more closely related to children’s word comprehension rather than production. They reasoned that if, as Gopnik and Meltzoff (1987) suggested, children reflect their understanding of categories by word production, then it follows that this knowledge would be first reflected in word comprehension which typically precedes production (Ingram, 1974). In support of this idea, Gershkoff-Stowe et al. (1997) additionally compared children who were “latetalkers,” with a small productive vocabulary, to children who were matched in age and also children who were matched in productive vocabularies. They found that the late-talkers reliably outperformed the vocabulary-matched children in a categorization task, but did not perform reliably differently than the age-matched children. This finding further supports General and Specific 6 the idea that comprehension, rather than production, is more closely associated with the emergence of advanced categorization behavior. Riccuiti, Thomas, and Riccuiti (1999) added further support to the claim that children’s comprehension of words is closely related to their categorization abilities. In their study, children were presented with an object manipulation task in which they were encouraged to sort common objects for which they had names (e.g. cup, shoe). They found that children’s advanced category behavior in this task was highly correlated with their knowledge of the labels for the common objects. Additionally, Waxman and Markow (1995) found that giving infants novel names for objects helped them categorize these objects. They suggested that labels may help infants form categories of objects, as giving several different objects the same label implies that they are the same “kind” of thing. Taken together, the literature presents contradictory evidence concerning what factors are related to changes in children’s categorization abilities. Is the emergence of advanced categorization abilities more closely related to children’s productive vocabulary, as Gopnik and Meltzoff (1987) and Poulin-Dubois et al. (1995) suggested? Or, is comprehension a better predictor of children’s categorization skills, as Gershkoff-Stowe et al. (1997) suggested? A related question, based on Riccuiti and colleagues’ (1999) findings, is how specific word knowledge is related to children’s ability to categorize. The purpose of the current study is to address these questions exploring whether understanding and labeling novel objects facilitates children’s ability to categorize those objects at different stages of productive vocabulary growth. Specifically, the present study compares the categorization skills of children during and after the vocabulary spurt using a standard object manipulation task (Riccuiti, 1965; General and Specific 7 Sugarman, 1981). In order to locate children who were either experiencing the vocabulary spurt, or who had already passed through this linguistic milestone, children from two age groups were initially recruited: 18-20 months, and 24-26 months of age. These age groups were selected on the basis on Goldfield and Reznick’s (1990) findings that most children experience the vocabulary spurt between 18-20 months, and further experience a decline in the rate of vocabulary growth at 24 months when children begin to put words together in sentences. Because age is not always an accurate predictor of vocabulary, once recruited on the basis of age, children were instead compared on the basis of productive vocabulary size. Children in the High Vocabulary group had greater than 150 words in their productive vocabulary, as measured by a parental checklist, while children in the Low Vocabulary group had less than 150 words in their productive vocabulary. We chose 150 words as a cut-off point because research has indicated that children who are beginning the vocabulary spurt have generally less than 150 words in their productive vocabulary (Goldfield & Reznick, 1990). All children were presented with an object manipulation task to assess their categorization skills. The objects presented to the children in the task were novel objects. To investigate the relationship between specific word knowledge and categorization, half of the children in each age group received novel names for the experimental objects prior to the object manipulation task (Name condition) and half of the children were exposed to but did not receive names (No Name condition). A comprehension task was used to obtain a measure of whether children in the Name condition understood the names assigned to the experimental objects. Children in the No Name condition were not tested on their understanding of the names, but were again presented with the novel objects in a General and Specific 8 comparable control task. Additionally, parents completed a detailed vocabulary checklist estimating the size of the child’s productive vocabulary. These vocabulary measures, yielded, first, a comprehension score of the novel words used in the study, and second, an overall productive vocabulary estimate. Taken together, these two measures allowed examination of the relationship between children’s word comprehension and production abilities and their categorization skills. As in previous research (Rakison & Butterworth, 1998; Sugarman, 1981), sequential touching was used as a measure of children’s category performance. The logic behind using sequential touching as a measure of categorization is that if children sequentially touch objects within a category more often than chance, it suggests that they recognize commonalties between the objects. That is, they see these objects as related by category (Mandler, Fivush, & Reznick, 1987). Two methods of examining categorization through sequential touching were used. First, I looked at whether children were systematically touching the objects within a category. Then, I examined the sophistication of children’s categorization by classifying children as single or dual categorizers. Children who were single categorizers only systematically touched objects from one category within an object set, while children who were dual categorizers systematically touched both categories of objects within the set. Dual categorization is a more advanced behavior, as it indicates that children are not only recognizing the commonalities between objects in one category, but also the differences between two categories of objects (Sugarman, 1981). I looked at how children’s categorization was related to their productive vocabulary as well as their comprehension of the individual experimental words. Three hypotheses were tested. First, in accordance with previous research (Poulin-Dubois et al, 1995), I General and Specific 9 hypothesized that children with larger productive vocabularies would show more advanced categorization than children with smaller productive vocabularies. However, following Riccuiti et al.’s (1999) finding that children’s knowledge of objects names was closely related to their ability to categorize them, I predicted that learning the specific names of individual objects would help children categorize them. Thus, I hypothesized, in addition, that children from both the High and Low vocabulary groups in the Name condition would outperform children in the No Name condition. This would lend support to the idea that children’s comprehension of words is more closely related to their categorization abilities than their productive vocabulary (Gershkoff-Stowe et al., 1997). Finally, I hypothesized that children with low vocabularies would benefit more from receiving names for the objects than children with larger productive vocabularies. I reasoned that children during the vocabulary spurt are just beginning to show advanced category performance, and labeling the objects might further highlight relationships between them, as Waxman and Markow (1995) suggested. Thus, I expected that children in the Name condition in the Low vocabulary group would outperform children in the Name condition in the High vocabulary group.